Compressor control for heat transfer system

ABSTRACT

A compressor is connected with an evaporator, a condenser, and an electrically controlled valve for circulating a working fluid in a system for recovering waste heat to provide heated water. A controller can be configured to adjust an operating capacity of the compressor to maintain output of a condenser temperature sensor at a condenser temperature set point, except when output of a compressor discharge pressure sensor indicates that a maximum operating pressure of the compressor has been exceeded. In such case the controller reduces the operating capacity of the compressor. The controller may further be configured to shut down the compressor when the discharge pressure sensor indicates that a shutdown pressure has been exceeded.

FIELD

The present invention relates to heat transfer systems.

BACKGROUND

It is known to employ energy exchange technologies in order to, for example, recover excess heat energy from an air-conditioning system to provide energy to heat water. Many examples of such heat-exchange technologies came about in the early 1980s which reflect the end of the energy crises of the 1970s. It is interesting to note that these heat-exchange technologies have not been generally adopted.

Existing solutions do not provide precise and robust control adequate for heat recovery systems, given that waste-heat recovery typically has large temperature gradients of the kind unforgiving to poor control.

SUMMARY

According to one aspect of the present invention, a heat transfer system includes a compressor for circulating a working fluid, the compressor having an inlet and an outlet. The compressor is operable at a controllable operating capacity. The system further includes a condenser connected to the outlet of the compressor, an electrically controlled valve positioned to receive working fluid from the outlet of the condenser, an evaporator connected between an outlet of the electrically controlled valve and the inlet of the compressor, a discharge pressure sensor located between the outlet of the compressor and the inlet of the electrically controlled valve, a condenser temperature sensor positioned to measure a temperature at the condenser, and a controller connected to the compressor, the discharge pressure sensor, and the condenser temperature sensor. The controller is configured to adjust the operating capacity of the compressor to maintain output of the condenser temperature sensor at a condenser temperature set point. The controller is further configured to reduce the operating capacity of the compressor when output of the discharge pressure sensor indicates that a maximum operating pressure of the compressor has been exceeded. The maximum operating pressure has a saturation temperature higher than the condenser temperature set point.

The controller can be configured to shut down the compressor when the output of the discharge pressure sensor indicates that a shutdown pressure has been exceeded, the shutdown pressure having a saturation temperature higher than the saturation temperature of the maximum operating pressure.

The system can further include a discharge temperature sensor located at the outlet of the compressor. The controller can be configured to incrementally close the electrically controlled valve when output of the discharge temperature sensor falls below a minimum discharge superheat temperature determined from output of the discharge pressure sensor.

The system can further include a subcooler connected between the condenser and the electrically controlled valve.

The condenser can be configured to receive flow of potable water to be heated.

The evaporator can be configured to receive flow of waste-heat bearing fluid.

According to another aspect of the present invention, a method of controlling a heat transfer system includes determining a condenser temperature at a condenser connected with an electrically controlled valve, an evaporator, and a compressor for circulating a working fluid. The compressor is operable at a controllable operating capacity. The method further includes adjusting the operating capacity of the compressor to maintain the condenser temperature at a condenser temperature set point, and reducing the operating capacity of the compressor when a discharge pressure of the compressor indicates that a maximum operating pressure of the compressor has been exceeded. The maximum operating pressure corresponds to a saturation temperature that is higher than the condenser temperature set point. The method further includes returning to adjusting the operating capacity of the compressor to maintain the condenser temperature at the condenser temperature set point after the discharge pressure of the compressor returns to below the maximum operating pressure.

The method can further include shutting down the compressor when the discharge pressure sensor indicates that a shutdown pressure has been exceeded. The shutdown pressure has a saturation temperature higher than the saturation temperature of the maximum operating pressure.

The method can further include incrementally closing the electrically controlled valve when a compressor discharge temperature falls below a minimum discharge superheat temperature.

The method can further include receiving flow of potable water to be heated at the condenser.

The method can further include receiving flow of waste-heat bearing fluid at the evaporator.

According to another aspect of the present invention, a heat transfer system includes a compressor for circulating a working fluid. The compressor has an inlet and an outlet and is operable at a controllable operating capacity. The system further includes a condenser connected to the outlet of the compressor. The condenser is configured to receive flow of water to be heated. The system further includes an electrically controlled valve positioned to receive working fluid from the outlet of the condenser, and an evaporator connected between an outlet of the electrically controlled valve and the inlet of the compressor. The evaporator is configured to receive flow of waste-heat bearing fluid. The system further includes a discharge pressure sensor located between the outlet of the compressor and the inlet of the electrically controlled valve, a condenser temperature sensor positioned to measure a temperature of the flow of water to be heated, and a controller connected to the compressor, the discharge pressure sensor, and the condenser temperature sensor. The controller is configured to adjust the operating capacity of the compressor to maintain output of the condenser temperature sensor at a condenser temperature set point, except when output of the discharge pressure sensor indicates that a maximum operating pressure of the compressor has been exceeded, in which case the controller reduces the operating capacity of the compressor.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate, by way of example only, embodiments of the present invention.

FIG. 1 is a diagram of a heat transfer system according to an embodiment of the present invention.

FIG. 2 is a pressure-enthalpy chart for the working fluid and the heat transfer system.

FIG. 3 is a block diagram of control logic of the controller.

FIG. 4 is a block diagram of decision logic of the controller.

FIG. 5 is a diagram of a heat transfer system according to another embodiment.

FIG. 6 is a diagram of a heat transfer system according to another embodiment.

FIG. 7 is a block diagram of control logic of the controller according to another embodiment.

FIG. 8 is a graph showing an example of compressor control.

DETAILED DESCRIPTION

FIG. 1 shows a heat transfer system 10 according to an embodiment of the present invention. The heat transfer system may be known as a heat pump, refrigeration loop, or similar. The heat transfer system provides precise and robust control, particularly when used in waste-heat recovery and water heating for human use.

The heat transfer system 10 includes a compressor 12, a condenser 14, an electrically controlled expansion valve 16, and an evaporator 18 connected together in a loop by fluid conducting piping. A working fluid is provided to the system 10. The working fluid can include refrigerants, such as R-134a, R-12, R-124a, R-401a, R-404a, R-409A, R-414A, or similar.

The compressor 12 is a screw-type compressor that circulates the working fluid in the system 10. The compressor 12 has an inlet for receiving working fluid in a low-pressure vapor state, and an outlet for discharging compressed working fluid as a high-pressure vapor. In other embodiments, the compressor is another kind of compressor.

The condenser 14 has an inlet connected to the outlet of the compressor 12, and has an outlet that feeds the electrically controlled valve 16. The condenser 14 can be configured to receive water or other fluid to heat. In this embodiment, cold water 22 flows into the condenser 14 and leaves the condenser 14 as hot water 24. For example, cold water 22 arrives at between 10 and 55 degrees Celsius and is heated to hot water 24 at between 40 and 70 degrees Celsius. Other temperatures are also possible. These example temperatures are conducive to heating water for residential or hotel use for cleaning, washing, cooking, or bathing. Cold water 22 may be potable and may originate from a municipal supply, from a re-circulating hot water tank, from a boiler feed line, or similar.

The electrically controlled valve 16 is positioned to receive at its inlet condensed working fluid from the outlet of the condenser 14. The electrically controlled valve 16 may be known as an ETX valve. The electrically controlled valve 16 can include a stepper motor and gear assembly configured to position a pin in the port through which working fluid flows, so as to incrementally open or close the port to increase or decrease flow of working fluid. The electrically controlled valve 16 creates a controllable pressure drop in the working fluid, thereby expanding the working fluid into a mixed vapor-liquid state at its outlet. Control of the valve 16 controls the pressure drop and thus the exiting quality, temperature, and pressure of the working fluid.

The evaporator 18 is connected between the outlet of the electrically controlled valve 16 and the inlet of the compressor 12. The evaporator 18 can be configured to receive a heat-bearing medium, such as water, an alternative liquid, or a gas. In this embodiment, waste-heat bearing fluid 26, such as that available from air-conditioning systems, enters the evaporator 18 and discharges its heat to the working fluid, before leaving the evaporator 18 as cooled fluid 28. The temperature of the arriving waste-heat bearing fluid 26 may be between about 10 and 50 degrees Celsius. Other temperatures are also possible.

The system 10 may further include a subcooler 32 connected between the condenser 14 and the electrically controlled valve 16. Flow of working fluid through the subcooler 32 may discharge heat to very cold water 34, having a temperature below the temperature of the cold water input to the condenser 14. Warmed water exiting the subcooler 32 may be fed into the condenser 14 as cold water 22.

The system 10 further includes a suction pressure sensor 42 located between the outlet of the electrically controlled valve 16 and the inlet of the compressor 12. In this embodiment, the suction pressure sensor 42 is located near the inlet of the compressor 12. The specific location of the suction pressure sensor 42 can be varied, provided that the pressure drop expected between the location of the suction temperature sensor 44 and the compressor 12 is taken into account.

The system 10 further includes a suction temperature sensor 44 located at the inlet of the compressor 12.

The system 10 further includes a controller 50 connected to the suction pressure sensor 42, the suction temperature sensor 44, and the electrically controlled valve 16. The controller 50 can include a processor, memory, input interface, and output interface. The controller 50 is configured to adjust the electrically controlled valve 16 to maintain output of the suction pressure sensor 42 and the suction temperature sensor 44 at levels above a saturation point of the working fluid.

FIG. 2 shows a pressure-enthalpy chart for the working fluid. No specific working fluid is depicted. However, the chart applies to at least those working fluids mentioned herein. Isothermals are shown in dashed line.

The controller 50 is configured to adjust (e.g., incrementally open or close) the electrically controlled valve 16 to maintain output of the suction temperature sensor 44 at a suction superheat temperature 62. To achieve this, a suction superheat set point 64 is set above the saturation point of the working fluid. Maintaining the suction superheat temperature 62 to be at the suction superheat set point 64 can prevent the evaporator 18 from overheating the working fluid, which may detrimentally affect output of the compressor 12 and cost compressor power, and may also prevent under-heating the working fluid, which can advantageously prevent liquid-state working fluid from entering the compressor 12.

The controller 50 determines compressor inlet saturation temperature from output of the suction pressure sensor 42 and subtracts the determined saturation temperature from the output of the suction temperature sensor 44 to determine the actual suction superheat temperature 62. The controller 50 employs a suction superheat control loop to maintain the suction superheat temperature 62 at the suction superheat set point 64 by controlling the electrically controlled valve 16. Example values for the suction superheat set point 64 include 3-5 degrees Kelvin, and similar values above saturation suitable for a safety margin above saturation. The suction superheat set point 64 is a differential temperature relative to the saturation temperature and so can be expressed in relative units such as Celsius or Fahrenheit or absolute units such as Kelvin or Rankine.

In operation, when the heat input from the waste-heat bearing water 26 decreases, the system 10 may tend to output lower temperature working fluid at the evaporator 18, which may bring the working fluid exiting the evaporator 18 towards a saturated state. The risk of saturation at the compressor inlet is reduced or prevented by the controller 50 maintaining the suction superheat temperature 62 at the suction superheat set point 64.

The controller 50 may also be configured to incrementally close the electrically controlled valve 16 to maintain the output of the suction pressure sensor 42 to below a maximum suction pressure 65. This can advantageously maintain the suction pressure below the suction pressure limit of the compressor, particularly when the temperature of waste-heat bearing water 26 is relatively high. The maximum suction pressure 65 can be expressed in units of pressure or as a maximum saturation temperature, with output of the suction pressure sensor 42 being converted to saturation temperature to allow comparison.

Referring back to FIG. 1, the system 10 can further include a discharge pressure sensor 46 located between the outlet of the compressor 12 and the inlet of the electrically controlled valve 16. In this embodiment, the discharge pressure sensor 46 is located near the outlet of the compressor 12. The specific location of the discharge pressure sensor 46 can be varied, provided that the pressure drop expected between the location of the discharge pressure sensor 46 and the outlet of the compressor is taken into account. The system 10 can further include a discharge temperature sensor 48 located at the outlet of the compressor 12.

The controller 50 can be further configured to incrementally close the electrically controlled valve 16 to maintain output of the discharge pressure sensor 46 and the discharge temperature sensor 48 at levels above saturation of the working fluid.

Referring again to FIG. 2, the controller 50 is configured to incrementally close the electrically controlled valve 16 to maintain output of the discharge temperature sensor 48 at a discharge superheat temperature 66 that is above a minimum discharge superheat temperature 68. This can advantageously maintain the discharge superheat, particularly on start-up when the system 10 is cold or when the suction pressure is high and the discharge pressure is low. Operating the compressor 12 too close to saturation at discharge can result in liquid-state working fluid entering the lubricating oil system of the compressor 12. This problem is particularly evident in semi-hermetic screw-type compressors, which permit working fluid to enter the oil separator and may allow working fluid to cool significantly at discharge. Thus, the compressor discharge is controlled by maintaining the discharge superheat temperature 66 at least a minimum discharge superheat temperature 68 amount above the saturation point of the working fluid.

The controller 50 determines compressor discharge saturation temperature from output of the discharge pressure sensor 46 and subtracts the determined saturation temperature from the output of the discharge temperature sensor 48 to determine the actual discharge superheat temperature 66. The controller 50 employs a discharge superheat control loop to maintain the discharge superheat temperature 66 at above the minimum discharge superheat temperature 68 by incrementally closing the electrically controlled valve 16. Example values for minimum discharge superheat temperature 68 include 20-25 degrees Kelvin, and similar values above saturation sufficient to prevent working fluid from cooling excessively inside the compressor 12 where it may contaminate lubricating oil and reduce the service life of the compressor 12. The minimum discharge superheat temperature 68 is a differential temperature relative to the saturation temperature and so can be expressed in relative units such as Celsius or Fahrenheit or absolute units such as Kelvin or Rankine.

Discharge pressure of the compressor 12 can be allowed to float based on control using the suction superheat set point 64, maximum suction pressure 65, and the minimum discharge superheat temperature 68. The controller 50 is thus configured to adjust the electrically controlled valve 16 to maintain evaporator 18 pressure as high as practical, while not exceeding the suction pressure limit of the compressor 12 and also while preventing saturated working fluid from condensing in the oil separator of compressor 12.

It can be seen from FIG. 2 that the system 10, when applied to waste heat recovery for heating residential or hotel water, operates at a relatively high end of the thermodynamic cycle for the working fluid. This allows efficient use of commonly available and safe working fluids to recover waste heat.

FIG. 3 illustrates control logic resident in the controller 50. The control logic can implement the methods and other techniques described herein. As such, the control logic may take the form of a specialized computer program, a group of parameters inputted into a preprogrammed control routine, or the like.

Output from the suction pressure sensor 42 is converted to a saturation temperature 82 at the inlet of the compressor 12. The can be performed with reference to a lookup table 84 that stores relationships between saturation pressures and saturation temperatures for the working fluid. The measured suction temperature from the suction temperature sensor 44 is reduced by the suction saturation temperature 82 to arrive at the actual suction superheat temperature 62.

The actual suction superheat temperature 62 and the suction superheat set point 64 are provided as inputs to a suction superheat control loop 88 whose output is a suction superheat valve command 90 for adjusting the electrically controlled valve 16. The suction superheat valve command 90 is a change in valve position that brings the actual suction superheat temperature 62 towards the suction superheat set point 64. The suction superheat set point 64 can be inputted by an operator of the system 10. The actual suction superheat temperature 62 and the suction superheat set point 64 can be expressed as true temperatures on a standard scale (e.g., 25 degrees Celsius) or as temperatures relative to saturation temperature (e.g., 5 degrees Celsius or Kelvin, or by convention “5K”). It is expected that such an incremental change in the valve 16 position is an incremental opening or closing of the valve 16.

Similarly, output from the discharge pressure sensor 46 is converted to a saturation temperature 92 at the outlet of the compressor 12 with reference to the lookup table 84. The measured discharge temperature from the discharge temperature sensor 48 is reduced by the discharge saturation temperature 92 to arrive at the actual discharge superheat temperature 66. The actual discharge superheat temperature 66 and the minimum discharge superheat temperature 68 are provided as inputs to a discharge superheat control loop 96 whose output is a discharge superheat valve command 98 for adjusting the electrically controlled valve 16. The discharge superheat valve command 98 is an incremental change in the valve 16 position that keeps the actual discharge superheat temperature 66 above the minimum discharge superheat temperature 68. It is expected that such an incremental change in the valve 16 position is an incremental closing of the valve 16. The minimum discharge superheat temperature 68 can be inputted by an operator of the system 10. The superheat temperatures 66, 68 can be expressed in a standard scale (e.g., 80 degrees Celsius) or as relative temperatures (e.g., 20K).

An operator-adjustable maximum suction pressure 65 and the output of the suction pressure sensor 42 are taken as inputs to a suction pressure control loop 102, which outputs a suction pressure valve command 104 representing an incremental change in the valve 16 position that keeps the measured suction pressure below the maximum suction pressure 65. It is expected that such an incremental change in the valve 16 position is an incremental closing of the valve 16.

The control loops 88, 96, 102 may each be PI, PID, or P feedback control loop that provides error output representative of an incremental valve opening or closing value. In this embodiment, the control loops 88, 96, 102 are PI feedback control loops.

Decision logic 106 determines which of the valve commands 90, 98, 104 to send to the electrically controlled valve 16 as the actual valve command 108. In this embodiment, the decision logic 106 selects the valve command 90, 98, 104 that requests the largest increment of closing. If no valve command 90, 98, 104 requests an incremental closing of the valve 16, then the control logic selects the suction superheat valve command 90. This results in the ignoring of any incremental open requests from the discharge superheat valve command 98 and the suction pressure valve command 104. In other words, the controller 50 incrementally adjusts the valve 16 based on the suction superheat set point 64, unless the discharge superheat temperature 68 falls below its minimum 68 or the suction pressure 42 exceeds its maximum 65, in which case the controller 50 incrementally closes the valve 16 by the maximum closing increment requested. That is, the suction superheat control loop 88 controls the valve 16, unless incremental valve closing is requested by either or both of the suction pressure control loop 102 and the discharge superheat control loop 96, in which case control of the valve is passed to the control loop 102, 96 or 88 requesting largest incremental amount of valve closing.

FIG. 4 illustrates an example embodiment of the decision logic 106, assuming that incremental close commands are represented by negative values and incremental open commands are represented by positive values. The lowest value out of the suction superheat valve command 90, the suction pressure valve command 104, and the discharge superheat valve command 98 is selected at 120. If the lowest value is not negative, as determined at 122, then the value of the suction superheat valve command 90 is taken, at 124, as the output valve command 108. If the lowest value is negative, then, at 126, the lowest value is taken as the output valve command 108 to control the valve 16 to incrementally close.

The control process illustrated in FIGS. 3 and 4 repeats in real time or near real time, as the system 10 operates.

The controller 50 may further provide for an alarm shutdown if any of the sensors 42-48 detects an abnormal condition on one of the control loops.

FIG. 5 illustrates another embodiment of a heat transfer system 130 according to the present invention. The system 130 is similar to the system 10 and only differences will be discussed in detail. For description of other features and aspects of the system 130, description of the system 10 can be referenced, with like numerals identifying like components.

The heat transfer system 130 uses two of the heat transfer systems 10, one a low-pressure system 134 to provide initial heating to water and another a high-pressure system 136 to provide further heating to the water.

The evaporators 18 may each receive waste-heat bearing fluid 26 and output cooled fluid 28. The subcoolers 32 may be fed in parallel with very cold water 32, which is warmed at 22 and then fed through the low-pressure system's condenser 14 before being fed through the high-pressure system's condenser 14, so that the water is progressively heated. Heated water 24 exits the first condenser 14 and further heated water 138 exits the second condenser 14.

A controller 132 controls operation of the low-pressure system 134 and the high-pressure system 136. The systems 134, 136 may use different working fluids and may be controlled at different pressures and temperatures. However, the principles of control are the same as discussed above.

The controller 132 operates using the teachings discussed herein for the control 50. That is, the controller 132 references the compressor suction temperature and pressure for each system 134, 136 to adjust the respective electrically controlled valve 16 to maintain the working fluid at the inlet of each of the compressors 50 at a respective suction superheat set point. At the same time, the controller 132 may reference compressor suction pressure for each system to incrementally close the respective electrically controlled valves 16 to maintain each suction pressure to below a respective maximum suction pressure. Further, the controller 132 may control the discharge temperature and pressure for each system 134, 136 to adjust the respective electrically controlled valve 16 to keep the working fluid at the outlet of the compressor 50 above a respective minimum discharge superheat temperature.

The suction superheat set points, the maximum suction pressures, and the minimum discharge superheat temperatures may be different or the same for each of the low-pressure system 134 and the high-pressure system 136. For example, the suction superheat set points and the minimum discharge superheat temperatures may be the same in both the low-pressure system 134 and the high-pressure system 136, while different maximum suction pressures may be used for the systems 134, 136. Other examples are also contemplated.

FIG. 6 illustrates another embodiment of a heat transfer system 150 according to the present invention. The system 150 is similar to the system 10 and only differences will be discussed in detail. For description of other features and aspects of the system 150, description of the system 10 can be referenced, with like numerals identifying like components. In addition, the system 150 can be used for each of the high- and low-pressure systems 136, 134 of FIG. 5.

The system 150 includes a condenser temperature sensor 49 located at the condenser 14. In this embodiment, the condenser temperature sensor 49 is located at the outlet of the condenser 14 in or near the flow of heated water 24. In other embodiments, the condenser temperature sensor 49 is located at other locations, such as inside the water-side of the condenser itself or in the flow of heated water 24 downstream of the condenser 14. Any of these locations, as well as other locations, can be considered as at the condenser 14.

The controller 152 references output of the condenser temperature sensor 49 to adjust the operating capacity of the compressor 12 to maintain the temperature measured by the condenser temperature sensor at a condenser temperature set point. Further, the controller 152 is also configured to reduce the operating capacity of the compressor 12 when the output of the discharge pressure sensor 46 indicates that a maximum operating pressure of the compressor 12 has been exceeded. The maximum operating pressure corresponds to a saturation temperature that is higher than the condenser temperature set point. Further, the compressor 12 may be shut down when output of the discharge pressure sensor 46 indicates that a shutdown pressure has been exceeded. The shutdown pressure corresponds to a saturation temperature that is higher than the saturation temperature of the maximum operating pressure. In the present embodiment, the controller 152 is able to keep the output of the discharge pressure sensor 46 below the shutdown pressure until the compressor reaches it minimum capacity, at which point the controller 152 may no longer be able to maintain the output of the discharge pressure sensor 46 below the shutdown pressure without turning the compressor off. In other embodiments, the electrically controlled valve 16 can be closed to reduce the compressor capacity in conjunction with or without the compressor's 12 internal capacity control.

The controller 152 can also be configured as discussed elsewhere herein to, for example, incrementally close the electrically controlled valve 16 when output of the discharge temperature sensor 48 falls below the minimum discharge superheat temperature 68, incrementally close the valve 16 when the maximum suction pressure 65 is exceeded, and adjust the valve 16 so that the suction superheat temperature 62 tracks the suction superheat set point 64.

FIG. 7 illustrates control logic resident in the controller 152. The control logic can implement the methods and other techniques described herein. As such, the control logic may take the form of a specialized computer program, a group of parameters inputted into a preprogrammed control routine, or the like. Control logic of the controller 152 can be combined with control logic of the controller 50 (FIG. 3).

Output of the condenser temperature sensor 49 and a condenser temperature set point 162 are provided as inputs to a water-heating control loop 164 whose output is an adjust capacity command 166 for adjusting the operating capacity of the compressor 12. The adjust capacity command 166 represents a change in compressor operating capacity that brings the measured condenser temperature towards the set point 162. For example, if the measured condenser temperature rises above the set point 162, then the compressor capacity is reduced. Conversely, if the measured condenser temperature drops below the set point 162, then the compressor capacity is increased. The condenser temperature set point 162 can be inputted by an operator of the system 150. The output of the condenser temperature sensor 49 and the condenser temperature set point 162 can be expressed as true temperatures on a standard scale (e.g., 60 degrees Celsius).

In this embodiment, a maximum operating pressure for the compressor 12 can be inputted as a saturation temperature operating limit 168. Output from the discharge pressure sensor 46 is converted to the discharge saturation temperature 92 with reference to the lookup table 84, which stores relationships between saturation pressures and saturation temperatures for the working fluid. The discharge saturation temperature 92 and the saturation temperature limit 168 are used as inputs by a compressor back-off control loop 170, which is configured to output an adjust capacity command 172 to keep the discharge saturation temperature 92 below the saturation temperature limit 168. In other embodiments, the compressor back-off control loop 170 operates on pressure values directly.

In addition, a shutdown pressure for the compressor 12 can be inputted as a shutdown saturation temperature 174. The discharge saturation temperature 92 and the shutdown saturation temperature 174 are used as inputs by a compressor shutdown check 176 which is configured to output a shutdown command 178 when the compressor 12 maximum safe pressure has been exceeded. In other embodiments, the compressor shutdown check 176 operates on pressure values directly.

The control loops 164, 170 may each be PI, PID, or P feedback control loop that provides error output representative of an incremental change in compressor capacity. The compressor shutdown check 176 can include a logical comparison that acts on inputted values and outputs a resulting value.

Decision logic 180 selects one of the adjust capacity commands 166, 172 and the shutdown command 178 to send to the compressor 12 as the actual compressor command 182. In this embodiment, the decision logic 180 is as follows. Presence of a shutdown command 178 always results in the compressor 12 being shut down. Otherwise, the adjust capacity command 172 from the back-off control loop 170 is selected if it represents the greatest amount of reduced capacity for the compressor 12, while the adjust capacity command 166 from the water-heating control loop 164 is selected at all other times. In other words, assuming that reducing capacity commands have negative values and increasing capacity commands are positive, if no shutdown command 178 is present, then the decision logic 180 selects the lowest negative value from the control loops 164, 170. If no negative value is available, then the decision logic 180 selects the positive value from the water-heating control loop 164. Thus, the compressor 12 is operated according to the water-heating control loop 164, unless further reduced capacity is demanded by the back-off control loop 170, and both of these control schemes are overridden by the shutdown check 176.

FIG. 8 shows an example of operation of the system 150 as controlled by the controller 152 when the system 150 is used for water heating.

Initially, the temperature of hot water 24 output by the condenser 14, as measured by the condenser temperature sensor 49, tracks the set point 162 of, for example, 60 degrees Celsius. The discharge saturation temperature 92 of the compressor 12 is somewhat higher (e.g., 64 degrees Celsius) at this time due to the temperature approach at the hot end which, is present to a larger or smaller degree in all heat exchangers, and the compressor 12 is running at full capacity.

After operating at steady state for a time, a disturbance occurs, such as a normal reduction in flow rate of cold water 22 into the condenser 14. Consequently, the hot water temperature 49 rises. The water-heating control loop 164 responds by reducing the compressor capacity. However, the hot water temperature 49 continues to rise. As a result, the compressor discharge saturation temperature 92 rises to exceed the saturation temperature limit 168 (e.g., 68 degrees Celsius). In response, the compressor back-off control loop 170 commands the compressor 12 to further reduce capacity until the discharge saturation temperature 92 is below the saturation temperature limit 168, thereby avoiding compressor shutdown at the shutdown saturation temperature 174 (e.g., 72 degrees Celsius).

A short time later, the flow rate of cold water 22 into the condenser 14 increases, and the temperatures 92, 49 drop enough so that control of the compressor 12 is returned to the water-heating control loop 164.

As can be seen, controlling the compressor 12 in this way prevents shutdown of the compressor 12. Further, use of the compressor back-off control loop 170 permits the operation of system 150 over a wider range of inlet conditions for cold water 22. By enabling system 150 to operate in this manner, the number of starts and stops of compressor 12 are reduced. This is advantageous as it provides more consistent temperature of hot water 24 and extends the life of compressor 12. Other advantages will be apparent to those skilled in the art.

In view of the above, it should be understood that the control techniques and systems described herein are precise, robust, and efficient, and particularly well suited for control of heat transfer systems used for waste heat recovery to heat water for human use in cooking, cleaning, bathing and other activities.

While the foregoing provides certain non-limiting example embodiments, it should be understood that combinations, subsets, and variations of the foregoing are contemplated. The monopoly sought is defined by the claims. 

What is claimed is:
 1. A heat transfer system comprising: a compressor for circulating a working fluid, the compressor having an inlet and an outlet, the compressor operable at a controllable operating capacity; a condenser connected to the outlet of the compressor; an electrically controlled valve positioned to receive working fluid from the outlet of the condenser; an evaporator connected between an outlet of the electrically controlled valve and the inlet of the compressor; a discharge pressure sensor located between the outlet of the compressor and the inlet of the electrically controlled valve; a condenser temperature sensor positioned to measure a temperature at the condenser; and a controller connected to the compressor, the discharge pressure sensor, and the condenser temperature sensor, the controller configured to adjust the operating capacity of the compressor to maintain output of the condenser temperature sensor at a condenser temperature set point, the controller further configured to reduce the operating capacity of the compressor when output of the discharge pressure sensor indicates that a maximum operating pressure of the compressor has been exceeded, the maximum operating pressure having a saturation temperature higher than the condenser temperature set point.
 2. The system of claim 1, wherein the controller is configured to shut down the compressor when the output of the discharge pressure sensor indicates that a shutdown pressure has been exceeded, the shutdown pressure having a saturation temperature higher than the saturation temperature of the maximum operating pressure.
 3. The system of claim 1, further comprising a discharge temperature sensor located at the outlet of the compressor, wherein the controller is configured to incrementally close the electrically controlled valve when output of the discharge temperature sensor falls below a minimum discharge superheat temperature determined from output of the discharge pressure sensor.
 4. The system of claim 1, further comprising a subcooler connected between the condenser and the electrically controlled valve.
 5. The system of claim 1, wherein the condenser is configured to receive flow of potable water to be heated.
 6. The system of claim 5, wherein the evaporator is configured to receive flow of waste-heat bearing fluid.
 7. A method of controlling a heat transfer system, the method comprising: determining a condenser temperature at a condenser connected with an electrically controlled valve, an evaporator, and a compressor for circulating a working fluid, the compressor operable at a controllable operating capacity; adjusting the operating capacity of the compressor to maintain the condenser temperature at a condenser temperature set point; reducing the operating capacity of the compressor when a discharge pressure of the compressor indicates that a maximum operating pressure of the compressor has been exceeded, the maximum operating pressure corresponding to a saturation temperature that is higher than the condenser temperature set point; and returning to adjusting the operating capacity of the compressor to maintain the condenser temperature at the condenser temperature set point after the discharge pressure of the compressor returns to below the maximum operating pressure.
 8. The method of claim 7, further comprising shutting down the compressor when the discharge pressure sensor indicates that a shutdown pressure has been exceeded, the shutdown pressure having a saturation temperature higher than the saturation temperature of the maximum operating pressure.
 9. The method of claim 7, further comprising incrementally closing the electrically controlled valve when a compressor discharge temperature falls below a minimum discharge superheat temperature.
 10. The method of claim 7, further comprising receiving flow of potable water to be heated at the condenser.
 11. The method of claim 10, further comprising receiving flow of waste-heat bearing fluid at the evaporator.
 12. A heat transfer system comprising: a compressor for circulating a working fluid, the compressor having an inlet and an outlet, the compressor operable at a controllable operating capacity; a condenser connected to the outlet of the compressor, the condenser configured to receive flow of water to be heated; an electrically controlled valve positioned to receive working fluid from the outlet of the condenser; an evaporator connected between an outlet of the electrically controlled valve and the inlet of the compressor, the evaporator configured to receive flow of waste-heat bearing fluid; a discharge pressure sensor located between the outlet of the compressor and the inlet of the electrically controlled valve; a condenser temperature sensor positioned to measure a temperature of the flow of water to be heated; and a controller connected to the compressor, the discharge pressure sensor, and the condenser temperature sensor, the controller configured to adjust the operating capacity of the compressor to maintain output of the condenser temperature sensor at a condenser temperature set point, except when output of the discharge pressure sensor indicates that a maximum operating pressure of the compressor has been exceeded, in which case the controller reduces the operating capacity of the compressor.
 12. The system of claim 11, wherein the controller is configured to shut down the compressor when the output of the discharge pressure sensor indicates that a shutdown pressure has been exceeded, the shutdown pressure being higher than the maximum operating pressure.
 13. The system of claim 12, further comprising a discharge temperature sensor located at the outlet of the compressor, wherein the controller is configured to incrementally close the electrically controlled valve when output of the discharge temperature sensor falls below a minimum discharge superheat temperature determined from output of the discharge pressure sensor. 